This invention relates to a method of processing semiconductor materials in order to create photonic band gap materials.
Semiconductor materials have an electrical band gap, which exists between the conduction band and the valence band, which electrons may not occupy due to an absence of energy levels. In a similar manner, there are some materials that exhibit a photonic band gap in which light of a given wavelength may not propagate. Such photonic band gap materials have great potential for use in constraining and trapping light for example in waveguides, optical memory, cavities of light emitting devices etc.
A number of methods of making photonic band gap crystals have been proposed; Lin et al, xe2x80x9cA three-dimensional photonic crystal operating at infrared wavelengthsxe2x80x9d, Nature, vol. 394, Jul. 16, 1998, pp 251-253, report a xe2x80x98woodpilexe2x80x99 crystal of polycrystalline silicon rods which was fabricated by depositing a layer of silica, masking the silica in a desired pattern, etching the unmasked silica and filling the trenches with polycrystalline silicon. The surface of the wafer was then made flat using chemical mechanical polishing and the process was repeated, with successive layers of polycrystalline silicon rods being formed in alternating orthogonal orientations. Once a sufficient number of layers had been deposited, the SiO2 was removed in an HF/H2O solution. The polycrystalline silicon rods had a thickness of 1.6 xcexcm, a width of 1.2 xcexcm and the spacing between rods was 4.2 xcexcm. The photonic band gap crystal had a stop band of 10-14.5 xcexcm with an attenuation of approximately 12 dB per cell.
Alternative techniques used to fabricate photonic band gap materials include the use of two photon excitation resins (Cumpston et al, xe2x80x9cTwo-photon polymerisation initiators for three-dimensional optical data storage and microfabricationxe2x80x9d, Nature, volume 398, Mar. 4, 1999, pp 51-54) and radio frequency bias sputtering of Si/SiO2 (Hanaizumi et al, xe2x80x9cPropagation of light beams along line defects formed in a Si/SiO2 three-dimensional photonic crystals: Fabrication and observationxe2x80x9d, Applied Physics Letters, volume 74, number 6, Feb. 8, 1999, pp. 777-779). For a more general description of photonic band gap materials and crystals see J D Joannopoulos, R D Meade, J N Winn, xe2x80x9cPhotonic crystals: Molding the Flow of Lightxe2x80x9d, Princeton University Press, ISBN 0-691-037447-2.
The disadvantage with these known techniques is that they are not compatible with fabrication techniques for opto-electronic devices, which will reduce the level of integration that will be possible. Additionally, techniques which rely on mechanical steps, such as polishing, will have problems when attempting to fabricate crystals which require physical dimensions having a very small resolution, for example sub-micron resolution.
According to a First aspect of the present invention there is provided a method of making a photonic band gap material, the method comprising the steps of;
(a) growing an epitaxial layer of a first semiconductor material onto a substrate;
(b) applying a mask to selected areas of the first semiconductor material and etching away the non-masked areas of the first semiconductor material to form a plurality of recesses;
(c) selectively growing an epitaxial layer of a second semiconductor material to fill the plurality of recesses created by the etching of the first semiconductor material; characterised in that the method comprises the further steps of;
(d) growing a further epitaxial layer of the first semiconductor material over the first semiconductor material and the second semiconductor material;
(e) applying a mask to selected areas of the further epitaxial layer of the first semiconductor material and etching away the non-masked areas of the further epitaxial layer of the first semiconductor material to form a further plurality of recesses, said further plurality of recesses being rotationally displaced with regard to the plurality of recesses formed within the preceding layer of the first semiconductor material;
(f) selectively growing a further plurality of epitaxial layers of the second semiconductor material to fill the recesses created by the etching of the first semiconductor material; and
(g) repeating steps (d), (e), and (f) as required to form a semiconductor product having a plurality of layers of interleaved regions of the first semiconductor material and the second semiconductor material, the regions in each of the layers being rotationally displaced with regard to the regions in the adjacent layers.
The advantage of this method is that the deposition and etching processes allow for very accurate control of the recesses and layers, giving significantly increased control over the dimensions of the structure. This enables semiconductor structures having a higher quality to be made. The deposition and etching processes are the same as those used in the fabrication of other semiconductor devices, enabling structures made according to the above to be readily integrated with other semiconductor devices.
Preferably the method is further characterised by the etching of the semiconductor product to selectively remove substantially all of the first semiconductor material whilst leaving the second semiconductor material substantially unaffected. Alternatively, the method is further characterised by the etching of the semiconductor product to selectively remove substantially all of the second semiconductor material whilst leaving the first semiconductor material substantially unaffected. The advantage of this is the increased difference in permitivity which is gained by removing one of the semiconductor materials will enhance the properties of the semiconductor product. The adjacent layers of semiconductor materials may be rotationally displaced by substantially 90xc2x0.
Preferably the first semiconductor material is indium gallium arsenide and the second semiconductor material is preferably indium phosphide. This enables the semiconductor structure to be made using well known semiconductor fabrication processes. Additionally, this has the advantage that the semiconductor structure can function in one of the telecommunications wavelength windows and that such structures can be integrated within opto-electronic components that operate within one of the telecommunications wavelength windows.
Preferably the indium phosphide is selectively grown in the presence of a chloride compound which gives the advantageous deposition of the indium phosphide is selectively grown in the presence of phosphorus trichloride (PCI3)
According to a second aspect of the present invention there is provided a photonic band gap material fabricated using a method as described above.
According to a third aspect of the present invention there is provided an opto-electronic device comprising a photonic band gap material fabricated as described above.